Abstract. We describe here the development and evaluation of an Earth system model
suitable for centennial-scale climate prediction. The principal new
components added to the physical climate model are the terrestrial and ocean
ecosystems and gas-phase tropospheric chemistry, along with their coupled
interactions.

The individual Earth system components are described briefly and the
relevant interactions between the components are explained. Because the
multiple interactions could lead to unstable feedbacks, we go through a
careful process of model spin up to ensure that all components are stable
and the interactions balanced. This spun-up configuration is evaluated
against observed data for the Earth system components and is generally found
to perform very satisfactorily. The reason for the evaluation phase is that
the model is to be used for the core climate simulations carried out by the
Met Office Hadley Centre for the Coupled Model Intercomparison Project
(CMIP5), so it is essential that addition of the extra complexity does not
detract substantially from its climate performance. Localised changes in
some specific meteorological variables can be identified, but the impacts on
the overall simulation of present day climate are slight.

This model is proving valuable both for climate predictions, and for
investigating the strengths of biogeochemical feedbacks.